Inductively coupled plasma mass spectrometry, denoted by ICP-MS, is currently the main technique used for analysis of trace elements, and also the preferred detection technique in liquid chromatography for speciation. Speciation is understood to mean the assaying of the exact chemical form in which an element is found in an analysis sample.
The main advantages of ICP-MS comprise:                a very high sensitivity;        the independence of the intensity of the signal detected with respect to the molecular structure generating the latter; and        the absence of the suppression by absorption of the signal detected by the salts of the chromatographic mobile phase, which is, on the contrary, the main problem in electrospray ionization mass spectrometry.        
The aforementioned characteristics make ICP-MS a potentially attractive technique for assaying trace compounds in microvolumes of biological samples such as, for example, the content of individual cells, vacuoles, or the “spots”, points or bands of gel obtained by gel electrophoresis, after separation by means of chromatography at nanoscale flow rates, less than 500 nl/min for HPLC (High Performance Liquid Chromatography) columns having an inner diameter less than or equal to 100 μm.
The major problem suffered by this technique is however the unacceptable lack of operational interface, capable of introducing, without any dilution, the liquid effluent to be analyzed at flow rates of less than 1 μl/min in an efficient manner, that is to say with 100% transport into the plasma torch. It should be noted, in particular, that the introduction of a diluent has the effect of greatly reducing the strength of the signal and the sensitivity of the measurement.
Standard ICP nebulizers currently operate at flow rates of around 1 ml/min. There are nebulizers that make it possible to nebulize liquid effluents at flow rates of several μl/min, but none of the latter make it possible to nebulize effluents at nanoscale flow rates.
By way of nonlimiting example, a nebulizer of this type has been described by Patent Application EP 1 081 487. Although designed to provide nebulization of a liquid effluent in a wide range of flow rates, the minimum flow rate of liquid effluent achieved is not less than 5 to 7 μl/min. Using several elemental flows, the aforementioned nebulizer moreover makes use of a nebulizing gas in a supersonic flow regime which, due to turbulence introduced, does not allow an optimal stability of the process and of the nebulizing flow rate to be obtained.
U.S. Pat. No. 5,752,663 describes a nebulizer that makes use of a nebulizing gas in a laminar flow regime in which the outer side wall of the inner tube is beveled to reduce turbulence in the nebulizing gas and to thus form droplets of liquid effluent, or aerosol, of similar size, a size having little dispersion. Although the low size dispersion of the drops appears satisfactory, the aforementioned nebulizer does not make it possible to achieve a stable nebulization of liquid effluent at a low flow rate, of less than 1 μl/min, due to the overall dimensions of the assembly and of the abrupt transition of the outer tube, in the vicinity of the outlet orifice of the liquid effluent, site of turbulence even in laminar regime.